This invention relates to a method for operating a top-and-bottom blown converter.
As is well known in the art, conventional top-blown converters which are also referred to as LD converters often experience operating difficulties due to the phenomenon that molten metal is ejected out of the converter, which is generally called slopping and which is attributable to excessive oxidation of molten metal by the top blowing oxygen gas. To eliminate such a problem, top-and-bottom blown converters were recently developed by taking advantage of the oxygen bottom blown converter and have been commercially used in iron works. The top-and-bottom blown converters are generally constructed by modifying existing top-blown converters. More specifically, referring to FIG. 10, the top-blown converter is modified by providing a plurality of bottom blowing tuyeres 2 extending through a converter bottom 1 which is removably secured to a converter housing 7. A top-blowing lance 3 is vertically inserted through a top opening of the converter. With this arrangement, jets 4 of oxygen gas are blown through the top-blowing lance 3 onto the surface of a molten metal bath 6 while an agitating gas 5 such as oxygen gas or argon gas is blown into the molten metal bath 6 through the tuyeres 2. This type of top-and-bottom blown converter is free of slopping since the molten metal is vigorously agitated by the bottom blowing gas to prevent the molten metal from being excessively oxidized with the top-blowing gas. Since a top-and-bottom blown converter constructed by modifying an LD converter, however, has a housing of a specific profile designed for the LD converter operation requiring top blowing only, the bottom blowing gas causes the molten metal to wave or vibrate and eventually the converter housing to severely vibrate, resulting in a variety of troublesome problems.
The problems involved in the conventional top-and-bottom blown converters will be described in more detail. Since the molten metal and hence the converter housing vibrate to a lesser extend in a normal top-blown converter operation than in an oxygen bottom-blown converter operation, a support for the housing of the top-blown converter is generally of a lesser strength as compared with that for the oxygen bottom-blown converter. On the other hand, the molten metal bath is vibrated to a large extent with the bottom blowing gas in the top-and-bottom blown converter. If a top-and-bottom blown converter is a modification of a top-blown converter having a housing support of a relatively low strength, the vibration of molten metal is transmitted to the converter housing so that the converter housing is severely vibrated. This vibration leads to several drawbacks, i.e. that the housing support is susceptible to fatigue failure and that the operation of the converter becomes unstable and, in some cases, the vibration is detrimental to the safety of the operators.
In order to minimize the above-mentioned vibration caused by the blowing of bottom-blowing gas, it is believed effective to locate bottom-blowing tuyeres remote from the center of the converter bottom and spaced apart from each other. However, in a top-and-bottom blown converter which is constructed by modifying a top-blown converter, it is difficult to distribute the bottom-blowing tuyeres in a spaced-apart relationship because of the housing profile and other factors. It is known that the life of tuyeres is considerably shortened if the tuyeres are washed with molten metal, that is, alternately exposed to air and molten metal during charging and tapping of molten metal. The oxygen bottom-blown converter has a housing profile approximating a spherical shape, that is, a housing profile having a reduced ratio H/D of housing height H to the maximum D of housing inner diameter, such that the bulge of a converter barrel serves as a reservoir for molten metal when the converter housing is tilted for charging or tapping of molten metal. Then the bottom blowing tuyeres are prevented from being washed upon charging or tapping even when they are spaced apart from each other and from the axis of the converter. However, a top-and-bottom blown converter which is constructed by modifying a conventional top-blown converter has the housing profile of the top-blown converter, that is, a vertically elongated profile approximating a rotary oval body having a less bulged barrel and an increased height-to-diameter ratio H/D. It thus is not possible to tilt the modified converter over a large angle for charging or tapping of molten metal. Since the lower portion of the converter barrel and the peripheral portion of the converter bottom constitute a metal reservoir upon loading and tapping of molten metal, the bottom blowing tuyeres should be collectively arranged on or in proximity of a line passing through the axis of the converter and parallel to the trunnion axis to prevent the bottom-blowing tuyeres from being washed with the molten metal upon charging or tapping thereof. The location of bottom blowing tuyeres is limited in a top-and-bottom blown converter which is constructed by modifying a top-blown converter, and it is very difficult in practice to reduce the vibration of a molten metal bath by arranging the bottom-blowing tuyeres in a spaced-apart relationship.
A primary object of the present invention is to minimize vibration of a molten metal bath in a top-and-bottom blown converter to thereby diminish vibration of the converter housing.
Taking into account the advantage of the top-and-bottom blown converter that the location of hot spots created by oxygen gas from a top-blowing lance need not be limited to the proximity of the axis of the converter because agitation of a molten metal bath is improved over the top-blown converter, and more specifically, the molten metal is agitated with the bottom-blowing gas to such a full extent that the top-blowing oxygen gas need not assist in agitating the molten metal, we determined the relationship of the location of hot spots to the vibration of molten metal by changing the top-blowing lance to vary the location of hot spots associated therewith. Finding that the magnitude of vibration of molten metal is closely related to the relative location of the hot spots and the bottom-blowing gas bubbling region, and more specifically, the vibration of molten metal can be minimized by designing the top-blowing multi-nozzle lance such that a hot spot created by a jet of oxygen gas from each of the nozzles of the lance is located outside the bottom-blowing gas bubbling region, we have achieved the present invention.
In conventional top-blown converters, it is a common practice to blow oxygen gas onto the surface of molten metal through a top-blowing lance thereabove for the purpose of effecting desiliconization, decarbonization and dephosphorization. The top-blowing lance usually has a plurality of, for example, three or four nozzles. A typical top-blowing lance is shown in FIG. 1 as having four nozzles 8 whose axis is at a small angle of about 8.degree.-10.degree. with respect to the axis of the lance 3. The angle of the nozzle axis with respect to the lance axis is referred to as nozzle inclination angle, hereinafter. A nozzle inclination angle on the order of 8.degree.-10.degree. is generally used in a multi-nozzle lance for conventional top-blown converters for the following reason. To increase the efficiency of decarbonization, it is required that the nozzle inclination angle be reduced to concentrate the associated oxygen jets within a relatively narrow region on the molten metal surface to allow the oxygen jets to impinge against the molten metal without dispersing their kinetic energy to reduce the kinetic energy per unit area. The concentrated energy causes the molten metal to be vigorously agitated. On the other hand, to promote slagging and dephosphorization, the nozzle inclination angle is desirably increased to cause part of the oxygen to be absorbed in a slag layer on the molten metal surface over a relatively large area. As a compromise between these contradictory requirements, the nozzle inclination angle is determined as described above. Furthermore, in conventional top-blown converters, the top-blowing lance is aligned with the axis of the converter for the purpose of rendering the molten metal reaction uniform and because of its location relative to the converter opening. The high temperature zones which are created on the molten metal surface by oxygen gas jets injected thereon through the top-blowing lance, that is the so-called hot spots, are located within a relatively narrow region which is confined around the axis of the converter on the basis of the nozzle inclination angle.
Nevertheless, the top-and-bottom blown converters constructed by modifying top-blown converters actually use the same lance as used in the top-blown converters although metallurgical effect, particularly molten metal agitating effect, is apparently different therebetween. As a result, the hot spots created by the top blowing gas from the lance are generally located within a relatively narrow region extending about the axis of the converter as in the case of the top-blown converters. On the other hand, the bottom-blowing tuyeres in the modified type of top-and-bottom blown converter must be collectively arranged on or in proximity of a line passing through the converter axis and parallel to the trunnion axis for the reason of tuyere life as described earlier. Consequently, the hot spots P created by the top-blowing gas from the lance 3 are located within a bubbling region of the molten metal surface which rising bubbles of the bottom-blowing gas from the tuyeres 2 reach (bottom-blowing gas bubbling region) or the hot spots P largely overlap the bottom-blowing gas bubbling region as seen from FIG. 10. When a conventional top-blowing lance was used in the top-and-bottom blown converter, it was difficult to locate the hot spots created by the gas from the top-blowing lance outside the bottom-blowing gas bubbling region.